Ion producing mechanism



p 1958 J. G. BAcKus 2,850,636

- ION PRODUCING MECHANISM Filed Aug. 29, 1945 INVENTOR. L/w/A/ 5 fiAc/rz/s WJW ATTORNEY.

United States Patent ION PRODUCING MECHANISM John G. Backus, Berkeley, Calif., assignor to the United States of America as represented'by the United States Atomic Energy Commission Application August 29, 1945, Serial No. 613,360

6 Claims. (Cl. 250-413) My invention relates to the art of separating a polyisotopic substance into segregable masses in each of which the naturally occurring distribution of the constituent isotopes has been radically altered so that at least one of the segregable masses obtained is enriched with respect to one of the isotopes. More particularly, my invention relates to the transmitter of a calutron.

The construction and theory of a calutron has been completely disclosed and set forth in an application for Letters Patent of the United States Serial No. 557,784, filed October 9, 1944, by Ernest 0. Lawrence, and issued as U. S. Patent No. 2,709,222 on May 24, 1955, and it would serve no useful purpose to repeat here the details of construction of a calutron. In passing, however, it may be stated that a calutron consists of an evacuated tank or vessel disposed between the poles of a powerful magnet and containing means for ionizing a vaporized polyisotopic substance and transmitting ions thereof in the form of a beam which follows an arcuate path within the tank. The action of the magnetic field upon the beam of ions is to cause ions of lighter isotopes to concentrate along the inner periphery of the beam, whereas ions of heavier isotopes congregate along the outer portion of the beam and follow a somewhat longer path. The beam of ions debouches into a collector which includes at least two compartments or pockets for the reception of segments of the beam. When the ions enter the collector they are neutralized. A mass richer in the concentration of the heavier isotope or isotopes than was the case with the original material will build up in the compartment receiving the outer portion of the beam.

The most important object of the present invention is to increase the density of the beam of ions in a calutron and thereby to increase the yield therefrom.

Another object of the invention is to increase the percentage of charge material ionized and projected through the calutron.

An important feature of my invention resides in a trimmer, or collimating plate, so disposed in the transmitter of a calutron as to intercept and block the emission of electrons from the filament except through .a concave path or ribbon extending through the ionizing chamber of the calutron transmitter. The trimmer serves to direct the electron stream closely adjacent and parallel to the boundary of the plasma in the ionizing chamber adjacent the exit slit.

These and other objects and features of my invention will be more readily understood and appreciated from the following detailed description of a preferred embodiment thereof selected for purposes of illustration and shown in the accompanying drawings, in which:

Figure 1 is a diagrammatic view of portions of a calutron;

Fig. 2 is a view in perspective of the calutron transmitter; and

Fig. 3 is a view in cross section along the line 3-3 of Fig. 2.

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Referring now to Fig. 1, there are shown, diagrammatically, the major elements of a calutron; namely, an arc block 10 provided with an internal ionizing chamber 12, and an exit slit 14 formed in one wall of the ionizing chamber 12. Adjacent the exit slit 14 there is disposed a pair of opposed accelerating electrodes 16. When the calutron is operated, there is formed Within the ionizing chamber 12 a plasma. For purposes of the present discussion, a plasma may be defined as a region electrically neutral but containing copious quantities of ions and electrons. The shape of the boundary of the plasma adjacent the exit slit 14 depends somewhat upon the character of the electric field between the accelerating electrodes 16 and the arc block 10. The voltage drop between thse two elements may be as great as 30 k. v., the accelerating electrodes 16 being negative with respect to the arc block 10. Under these conditions, the boundary of the plasma is concave with respect to the accelerating electrodes-16, the boundary being represented in Fig. 1 by the line 18.

It is to be understood that there is provided an electronemissive element which projects a stream of electrons through the ionizing chamber at right-angles to the plane of the paper. Inasmuch as the calutron is interposed between the poles of a powerful magnet, there is also present a strong magnetic field in which the direction of the flux is also perpendicular to the plane of the paper upon which the drawing has been made. One function of the magnetic flux is to bend the ion beam until it follows the curved path shown in Fig. 1, but another function of the magnetic flux is to collimate the electron stream so that the electrons emitted by the cathode do not fiy ofi at random but are'projected in straight lines across the ionizing chamber. The beam of ions is denoted generally by the reference numeral 20 and, as will be seen, terminates in a collector 22 which functions in the manner above described.

It is obvious that the source of the beam cannot be' a mathematical line since the ions which compose the beam are derived from several points across the face of the plasma boundary 18. Consequently, the beam 2% is really composed of a number of individual rays, three of which are diagrammatically represented by the lines 2?, 24, and 26. Measurements have established, however, that the beam 20, or the individual rays composing the beam 29, passes through avirtual focus point 28 disposed between the accelerating electrodes 16 and the arc block 16. When the rays 23, 24, and 26 are magnetically projected through the point 28, they appear to become asymptotic, which correlates with the concave configuration of the boundary of the arc plasma. It has also been established that the majority of the ions leaving the plasma do so at rightangles to the plasma boundary.

In view of the foregoing factors, I deemed it advantageous to devise a calutron transmitter in which the electron stream would be concentrated along the boundary of the plasma, since the ion density is always greatest in the immediate vicinity of the electron stream. By so directing the'electron' stream that it would correspond to the configuration of the plasma boundary, 1 increased the ion density in the plasma at the concave boundary thereof.

To achieve the desirable result referred to in the fore going paragraph, I constructed a calutron transmitter of the type shown in Figs. 2 and 3. I provided an arc block 40 consisting of an elongated casting having a chamber 42 for the reception of the charge material of polyisotopic substance, and an ionizing chamber 44 communicating with the charge chamber 42 by means of a conduit 46 formed by boring a hole through the Wall separating the chambers 42 and 44. The ionizing chamber 44 is open at one end and the opening is partiallyrestricted by means of a pair of face plates 48 of carbon or graphite, which form between them a relatively narrow exit slit. Supported above the arc block '40 is a pair of stiff, water-cooled leads 50, across. the ends of which is mounted a flat ribbon cathode or filament 52 ofthoriated tungsten or other, suitable el'ctron-emissive material. Supported beneath the filament 52 ('bymea'ns of.struc: ture'not shown in the drawing) is'aflat metal plate 54 insulated from. the arc block 40' and provided with a through-and-through crescent shaped aperture. or slit 56. The filament 52 and the plate 54 are so disposed that the crescent slit 56 lies directly beneath'the ribbon fila ment 52, and it should be emphasized that the filament is both longer and broader than the slit 56. The slit 56 is also. disposed directly above the ionizing chamber 44 and located just behind the plane of the exit slit formed by the face plates 48. i i i i The are block 40 itself serves as an anode with re- 7 spect to the cathode 52. The elements are so arranged in the calutron tank that the slit formed by the face plates 48 is parallel to the direction of the flux. In Fig. 2 the flux direction is indicated by the arrow 64. The magnetic field collimates the electron stream leaving the fila- 7 ment 52 so that all electrons traveling away from the filament do so along paths parallel to the magnetic flux.

'The plate 54 intercepts and blocks ofi all'electrons leav ing the filament 52 in downward direction with the exception of those which travel through the slit 56, and it will be evident that the combination of the slit 56 and the action of the magnetic flux is to form an electron cise'means so employed forming no novel portion of this invention; In Fig. 3 the plasma present in' the ionizing chamber 44 is designated by the stippled area and the Having now described and illustrated a preferred em: bodim'ent of my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. A calutron transmitter comprising'walls defining a chamber having an elongated exit slit in one wall there of, an electrode disposed outside said chamber in position to withdraw ions through said slit, a flat ribbon cathode disposed adjacent one end of saidchamber, and a plate having a crescent shaped slit therethrough, said plate being disposed between the said chamber and said cathode in such position that. access from the cathode to the chamber is blocked save through said crescent shaped slit. I 7

2. An ion producing mechanism comprising a housing defining a chamber having an opening, an electrode disposed adjacent the opening for withdrawing ions therethrough, an electron emitting surface adjacent one end of said chamber, and a plate disposed between the elecposed adjacent one end of said chamber, anda trimmer plate disposed between said cathode and .said' chamber and having a crescent shaped slot therethrough smaller in area than thearea of the cathode for definingthe shape of an electron streamprojected from said cathode across said chamber.

4. A calutron transmitter comprising walls defining a chamber havingan elongated exit slit, an electrode disposed adjacent said slit for withdrawing ions from said chamber, 'a cathode disposed adjacent one end of said chamber, and a trimmer plate disposed between said chamber and said cathode and having therein a crescent boundary of the plasma adjacent the exit slit is indicated by the dotted line 60. Adjacent the exit formedby the face plates 48 there is also shown, in Fig. 3, a pair of accelerating electrodes 62 disposed in advance of the arc block 40. The relative dispositions of the plasma boundary 60 and the electron stream is shown in Fig. 3 wherein the dotted lines 63 represent a cross section through the electron stream. 7

In view of the discussion of Fig. 1, it will be evident that I have succeeded in directing through the arc chamber 44 a stream of electrons which closely parallels the boundary of the arc plasma and thereby greatly increases the ion density in the region closely adjacent'the boundary of the plasma. It has been found that by so increasing the ion density, a much greater number of ions is withdrawn from the plasma by the attraction of the acrcelerating electrodes 62, consequently increasing the density of the ion beam projected through the calutron. Furthermore, the combination of the crescent-shaped slit 56 beneath the filament, the accelerating electrodes, and the juxtaposition of the plasma boundary'and the elec: tron stream increases the percentage of the charge material which is ionized and projected through the calutron.

It is to be understood that in normal practice the charge chamber 42 is to be filled with polyisotopic material capable of being vaporized by means 'of heating elements (not shown). The vapor passes into the ionizing chamber44 where ionization of the vapor is accomplished by the impact of the electrons from the filament upon the particles of the vapor, as well as by collision and c onsequent secondary emission of electrons within the ionizingchamben shaped slot, said trimmer plate being disposed in such position that the slot therein provides access for, electrons leaving said cathode, wherebyrelectrons are pro-' through, an electron emitting surface adjacent one end of said chamber, a plate having a: crescent-shaped aperture therein for producing a crescent-shapedarc through said chamber, and means for maintaining said plate at the potential of, said electron emitting surface. 1 6. Ion producing mechanism disposed in a uniform magnetic field and comprising a source block defining a chamber therein, said source block having an elongated slit formed therein parallel to said magnetic fieldand communicating between said chamber and the exterior of said source block, said source block further having an aperture therein communicating betweensaid chamber and the exterior of said chamber 'adjacentone'end of said slit, a pair of ion accelerating electrodes disposed on opposite sides of said slit exterior to said source block, a ribbon-like filament disposed adjacent said aperture outside of said chamber for-producing an arc discharge through said chamber parallel to and adjacent said slit, and attrimmer plate disposed intermediate said filament and source block and having a crescent shaped ape rture therein with the concavity thereof facing said slitwhere by the arc discharge is constrained to assume a; crescent shaped cross section and to lieimmediatelyadjacentand parallel to said slit, thereby producing an intensified and controlled ion distribution within the influence of said 'ion accelerating electrodes.

(References on following page) r 6 References Cited in the file of this patent 2,373,151 Taylor Apr. 10, 1945 UNITED STATES PATENTS 2,439,504 Br w y P 1948 2,212,827 Etzrodt Aug. 27, 1940 FOREIGN PATENTS 2,221,467 Bleakney Nov. 12, 1940 5 455,351 Great Britain Oct. 16, 1936 2,339,833 Bedford Jan. 25, 1944 

